Cloning, functional expression, and characterization of a chalcone 3-hydroxylase from Cosmos sulphureus

A chalcone 3-hydroxylase (CH3H) cDNA clone was isolated and characterized from Cosmos sulphureus petals accumulating butein (2′,3,4,4′-tetrahydroxychalcone) derivatives as yellow flower pigments. The recombinant protein catalyses the introduction of an additional hydroxyl group in the B-ring of chalcones, a reaction with high similarity to the hydroxylation of flavonoids catalysed by the well-studied flavonoid 3′-hydroxylase (F3′H). CH3H shows high specificity for chalcones, but a low F3′H activity was also detected. By contrast, the common F3′H from C. sulphureus does not accept chalcones as substrates and is therefore unlikely to be involved in the creation of the B-ring hydroxylation pattern of the yellow flower pigments. CH3H was primarily expressed in young buds, the main tissue for chalcone pigment formation. Expression levels in open flowers and 3-d-old seedlings were lower and almost no CH3H expression was observed in leaves. F3′H, in contrast, showed the highest expression also in buds, but comparable expression rates in all other tissues tested. Recombinant hybrid proteins constructed from CH3H and F3′H fragments demonstrated that amino acid residues at a substrate recognition site and an insertion of four amino acid residues in a putative loop region have an impact on chalcone acceptance. This is the first identification of a CH3H cDNA from any plant species.